1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery.
2. Description of the Prior Art
In recent years, as commercial cell phones, portable devices, personal digital assistances or the like have rapidly been diversified and reduced in size and weight, there is a strong need for development of a secondary battery as a power source thereof which is compact and lightweight and which also has a high energy density and can repeatedly charge and discharge over a long period of time. Specifically, a non-aqueous electrolyte secondary battery such as a lithium secondary battery has been studied intensively, because such secondary battery is excellent in terms of its size, weight and its high energy density over with a lead battery or a nickel cadmium battery which uses an aqueous electrolyte solution.
As a positive active material for the non-aqueous electrolyte secondary battery, various compounds have been investigated including titanium disulfide, vanadium pentoxide, and molybdenum trioxide, as well as other compounds having general formulae LixMO2 (where M is one or more transition metals) such as lithium cobalt complex oxides, lithium nickel complex oxides, and spinel type of manganese oxides. Particularly, lithium cobalt complex oxides, lithium nickel complex oxides, or spinel type manganese oxides can achieve a battery which provides a high discharge voltage by using such oxides as a positive electrode, because charge and discharge are performed at a noble potential than 4V vs. Li/Li+.
As a negative active material for the non-aqueous electrolyte secondary battery, various compounds have been investigated such as metallic lithium, lithium alloys, and carbon materials capable of intercalating and deintercalating lithium. Particularly, when using the carbon materials, there are some advantages that a battery having a long cycle life and a high level of safety can be obtained.
As an electrolyte for the non-aqueous electrolyte secondary battery, an electrolyte solution, produced by dissolving a supporting salt such as LiPF6 or LiBF4 in a mixed solvent including a solvent having a high dielectric constant such as ethylene carbonate or propylene carbonate and a solvent having a low viscosity such as dimethyl carbonate or diethyl carbonate, has generally been used.
The non-aqueous electrolyte secondary batteries are used in various circumstances. One of such circumstances is a cold climate. For example, a non-aqueous electrolyte secondary battery used for a cell phone is required to have excellent discharge characteristics even at a low temperature, since such cell phones may be used on skiing grounds or in mountains during a winter season.
The low temperature discharge characteristics are affected by various factors such as a current collecting property of the battery, an electric conductivity of the non-aqueous electrolyte, and a reactive resistance at positive and negative electrodes. Particularly in the case of a non-aqueous electrolyte secondary battery which generates a high voltage, components of the non-aqueous electrolyte are decomposed on a surface of an electrode, polymerized products which may cause a reduction in the electric conductivity of the electrolyte solution are produced, and a film having a low lithium ion conductivity is formed. Thus, the low temperature discharge characteristics of the non-aqueous electrolyte secondary battery have often been impaired.
As a method for preventing such decrease in the low temperature discharge characteristics induced by the decomposition of the components in the non-aqueous electrolyte, a small amount of additive is added to the non-aqueous electrolyte. For example, high temperature storage characteristics can be improved while also improving low temperature characteristics thereof by adding a small amount of vinylene carbonate to a non-aqueous electrolyte of the non-aqueous electrolyte secondary battery, as described in Japanese Patent Laid-Open No. 2002-15768. This method improves the low temperature discharge characteristics, but its effect becomes insufficient in these days because performance of the non-aqueous electrolyte secondary battery is required to be improved more than ever, so that a method for further improving the low temperature discharge characteristics has been desired.
In addition, a method of adding a cyclic sulfate to a non-aqueous solvent is described in Japanese Patent Laid-Open Nos. 10-189042and 11-162511. However, in such patent applications, there are no descriptions about additive amounts which are sufficient for improving the low temperature discharge characteristics, and also, there are no descriptions that it is possible to obtain especially excellent discharge characteristics at a low temperature by using a predetermined amount of vinylene carbonate mixed with the above described solvent.